1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to a magnetron driving control apparatus of a microwave oven and method thereof adapted to control driving of a magnetron according to voltages of a high voltage transformer and the magnetron in response to variations of foodstuffs to optimally cook the foodstuffs.
2. Description of the Prior Art
Generally, a microwave oven according to the prior art includes, as illustrated in FIGS. 1 and 2, a body 1, a cooking chamber with a door 3 hinged thereto, a key input unit 40 for setting up cooking courses such as cooking function, cooking time, cooking start/stop and the like and a display unit 4 for displaying the cooking courses.
Furthermore, the body 1 is formed therein with a diaphragm 5 for driving the cooking chamber 2 and a waveguide 6 is welded to the diaphragm 5. The waveguide 6 is disposed at one side thereof with a magnetron 30 and an opening 7 through which microwaves generated from the magnetron 30 are eradicated into the cooking chamber 2.
Meanwhile, FIG. 3 is a schematic block diagram for illustrating a microwave oven thus constructed according to the prior art, where the microwave oven includes a relay unit 8, a high voltage transformer 10, a high voltage diode 20, a high voltage capacitor 25, a magnetron 30, a key input unit 40, a weight sensor 42, a gas sensor 44 and a control unit 50.
In FIG. 3, the relay unit 8 is a relay for supplying or cutting off a commercial alternating current AC voltage (by way of example, 220-240 V) turned on or off by a control signal of the control unit 50 to thereafter be supplied via input terminals A and B, and the high voltage transformer 10 for transforming the commercial AC voltage to a high voltage (approximately 2,200 V) and a low voltage (approximately 3.4 V) includes a first coil 12 for inputting the commercial AC voltage from outside, a second coil 47 for outputting the high voltage and a filament coil 16 for outputting a low voltage.
The high voltage diode 20 and the high voltage capacitor 25 are disposed between the high voltage transformer 10 and the magnetron 30, which receives a lower voltage to be heated and also receives a direct current DC type of high voltage via the high voltage diode 20 and the high voltage capacitor 25 to generate microwaves.
Furthermore, the key input unit 40 outputs a key signal corresponding to the cooking courses and the weight sensor 42 detects the weight of foodstuff and outputs a detected signal corresponding thereto.
The gas sensor 44 detects the amount of gas generated from the foodstuff being cooked and outputs a detected signal corresponding thereto. The control unit 50 outputs a control signal for driving the magnetron 30 according to the key signal from the key input unit 40 and simultaneously counts a cooking time and establishes a cooking time according to the signals detected from the weight sensor 42 and the gas sensor 44, and when the counted cooking time is above the established cooking time, outputs a control signal for stopping the drive of the magnetron 30.
In the microwave oven thus constructed according to the prior art, when a user inserts foodstuff into the cooking chamber 2 and establishes a cooking course via the key input unit 40 to input a cooking start, a key signal corresponding thereto is output from the key input unit 40 to the control unit 50 and the control unit 50 outputs a control signal for driving the magnetron to the relay unit 8 and counts the cooking time at the same time.
Successively, the commercial AC voltage is supplied to the first coil 12 of the high voltage transformer 10 to thereby cause the filament coil 16 to be applied with a low voltage and the second coil 14 with a high voltage at the same time.
The filament is pre-heated and the high voltage is transformed to a high voltage of DC type via the high voltage capacitor 25 and the high voltage diode 20, and the DC-type high voltage is supplied to two poles of the magnetron 30 to thereby cause the magnetron 30 to generate microwaves. The microwaves are radiated into the cooking chamber 2 via the opening 7 of the waveguide 6 to heat and cook the foodstuff therein.
At this time, a detected signal corresponding to the weight of the foodstuff is input from the weight sensor 42 to the control unit 50 and a detected signal corresponding to the amount of gas generated from the foodstuff is input from the gas sensor 44 to the control unit 50.
The control unit 50 now discriminates the weight of the foodstuff and the amount of gas generated from the foodstuff according to the detected signals from the weight sensor 42 and the gas sensor 44 and compares the discriminated weight and amount of gas with pre-established weight and amount of gas. The control unit 50 establishes a cooking time according to the compared result.
Furthermore, the control unit 50 discriminates whether the counted cooking time is above the established cooking time, and if the counted cooking time is above the established cooking time, the control unit 50 outputs a control signal to the relay unit 8 to stop driving the magnetron 30.
The commercial AC voltage supplied to the high voltage transformer 10 is now cut off by the control signal from the control unit 50, thereby stopping driving the magnetron 30 and completing the whole cooking operations of the microwave oven.
Meanwhile, let's assume that an output power of the magnetron 30 is P.sub.in, and a power at a predetermined position in the cooking chamber 2 is P.sub.out, the P.sub.out can be obtained by following formulae 1, 2 and 3. EQU P.sub.in =E.sub.s.sup.2 Formula 1 EQU E.sub.y =E.sub.s sin(.chi.) Formula 2 EQU P.sub.out =(E.sub.y).sup.2 ={E.sub.s sin(.chi.)}.sup.2 =E.sub.s.sup.2 sin(.chi.).sup.2 Formula 3
Where, E.sub.s is a field energy formed by microwaves generated from the magnetron 30, that is, input field energy, and E.sub.y is a field energy at the predetermined position in the cooking chamber 2, that is, output field energy.
Accordingly, the output power of the magnetron 30 is obtained by a squared value of E.sub.s, the field magnitude formed by the microwaves generated from the magnetron 30.
At this time, the microwaves generated from the magnetron 30 are sine waves, so that the field energy E.sub.y at a particular position in the cooking chamber 2 is the field energy E.sub.s multiplied by sin(.chi.), where sin(.chi.) is varied in value or phase thereof according to the states of foodstuff (by way of example; kind, quantity, cooking process status of the foodstuff and the like).
In other words, absorbed quantity of microwaves differs according to the kind and quantity of the foodstuff and gas quantity, such that the output power P.sub.out at the particular position in the cooking chamber 2 differs according to the kind and quantity of the foodstuff. Particularly, because the quantity of gas generated from the foodstuff according to the cooking process status differs, the P.sub.out is also changed variably according to the cooking process status.
For reference, impedance characteristic of the waveguide 6 according to the quantity of foodstuff can be described in the polar chart illustrated in FIG. 4, where the Voltage Standing Wave Radio VSWR, that is, the impedance of the waveguide 6 is decreased when load is water of 2,000 cc under the microwave frequency of 2.44-2.47 GHZ, to thereby increase the output of the magnetron, whereas the impedance of the waveguide 6 is increased to decrease the output of the magnetron 30 when the load is water of 100 cc.
However, there is a problem in the conventional microwave oven thus constructed in that a cooking time has been controlled simply according to weight of the foodstuff and quantity of gas detected by a weight sensor and a gas sensor without consideration of output variations of a magnetron in response to cooking process status, such that the foodstuff has not been optimally cooked when the output of the magnetron is changed according to the cooking process status.
By way of example, when the output of the magnetron is decreased by quantity of gas that varies in response to the cooking process status, cooking is terminated with foodstuff not fully cooked or with soup not fully boiled.
When the output of the magnetron is over-fed, there is a problem in that the foodstuff is over-cooked or the soup is boiled dry.